Coal-fired power plants have long been known to cause acid rain and atmospheric pollution. It has been well known that to reduce these undesirable forms of pollution, the power plants have been forced to reduce ambient sulfur dioxide emissions. This reduction of emissions has been accomplished by either switching to low-sulfur coal or using various flue gas desulfurization(FGD) processes to reduce emissions from high-sulfur coals.
Most power plants utilizing high-sulfur coal(i.e. 3% by weight sulfur) employ either throw-away or regenerative processes. Throw-away processes involve various limestone injection or scrubbing techniques(dry or wet). They produce gypsum as a by-product which must be disposed of properly. Regenerative processes use sodium hydroxide scrubbing which is regenerated, resulting in sulfur dioxide which may be sold as such or converted to sulfuric acid. These processes are all expensive to build and operate. Thus, utilities often switch from high to low-sulfur coal(containing less than 1% by weight sulfur) in order to reduce sulfur dioxide emissions. However, using low sulfur coal confronts power plant operators with additional problems. When low-sulfur coal is burned, the fly-ash produced is too resistive. Thus, electrostatic precipitators(ESPs) that are designed to operate with fly-ash originating from high-sulfur coal burners do not work. In the new plants, the use of ESPs designed for the low-sulfur coal fly-ash would be very large in size and uneconomical.
Other solutions to these problems involve sulfur trioxide injection into the flue gas after the boiler and before the electrostatic precipitator. Ammonia injection or a combination of ammonia and sulfur trioxide have also been used. This process has been referred to as fly-ash conditioning or flue gas conditioning. Most fly-ash is composed of silica and alumina mixed with other metal oxides. Any polar compound which is sufficiently reactive and able to change the surface properties will render the ash less electrically resistive. While sulfur trioxide injection can condition the flue gas and decrease fly-ash resistivity to a level comparable to that obtained when high sulfur coal is used, the process is expensive. Either liquid sulfur dioxide or elemental sulfur is required. A catalytic reactor is needed to convert the sulfur dioxide to sulfur trioxide. The catalyst(V.sub.2 O.sub.5) life is limited, so that it must be periodically replenished. Ultra-high purity sulfur and filtered air must be used for a trouble-free operation. Sulfur must be kept molten, therefore, steam-jacketed piping is required. Systems in which sulfur oxides and water are present inevitably invite corrosion and require constant attention and maintenance. In-line spare pumps and air blowers are also necessary. These components result in expensive plant installation to generate sulfur trioxide as needed.
Several U.S. patents have been concerned with the ultraviolet treatment of flue-gases. For example, the photolytic oxidation of sulfur dioxide to sulfur trioxide, aided by UV radiation is demonstrated in U.S. Pat. No. 3,984,296 to Richards; 4,097,349 to Zenty; and 5,138,175 to Kim et al. The patents above teach flue gas treatment by a well-known photochemical process involving UV radiation to generate highly oxidizing species that attack target molecules in the flue gas. They may differ on the postulated reaction mechanisms, but they all claim to be processes that remove or help remove gaseous pollutants from the flue gases. However, none of the patents discussed above describe methods based on hydroxyl radical reaction engineering and photo-system design. None of these prior art references describe the importance of using water or water vapor solutions in a treatment process and most importantly and fundamentally, they do not claim interactions with fly ash.
U.S. Pat. No. 4,097,349 to Zenty describes oxidation of NO.sub.x,SO.sub.2, and hydrocarbons with UV radiation having a wavelength of from 240 nanometers to 340 nanometers. Equation 17 of the Zenty U.S. Pat. No. 4,097,349 depicts the absorption of 290-340 nanometers UV radiation to generate a singlet .sup.1 SO.sub.2. Equation 18 of the Zenty patent describes the absorption of 340-400 nanometer UV radiation by SO.sub.2 to produce triplet .sup.3 SO.sub.2. According to Zenty, through a series of steps, the singlet excited .sup.1 SO.sub.2 can be transformed to the triplet state, .sup.3 SO.sub.2. The excited triplet state can be chemically quenched with another species present in the gas stream such as nitrogen, oxygen, water, carbon dioxide, carbon monoxide, ozone, methane, and other hydrocarbons. Zenty's patent is essentially based on the aforementioned photo-processes and their consequences in photo-oxidation of NO.sub.x and SO.sub.2. The Zenty patent does not describe any specific mechanisms such as: free radical chain reactions, in general, and hydroxyl radical formation, in particular. For example, Zenty discusses the importance of hydroxyl radical (OH.) reactions (column 2, lines 65-68). In the presence of moisture, hydroxyl radical reactions dominate SO.sub.2 conversion, as shown in equations 13-16 of the Zenty patent. Fundamentally, the Zenty process does not deal with fly ash conditioning.
U.S. Pat. No. 3,984,296 to Richards describes a process for the reduction of sulfur and nitrogen oxide contaminants in effluent gas streams. Richards' patent teaches the formation of electron donor-acceptor molecular complexes (EDA complexes) in the flue-gas by exposure to lewis acids or bases generated electrostatically within a corona precipitator. Also, Richards describes a photochemical technique for the production of the EDA complexes using infrared radiation of 400 to 1,000 nanometers or UV radiation of 120-240 nanometer wavelengths.
Furthermore, Richards describes a technique for photo-induced oxidation of the EDA complexes and reaction of SO.sub.2 and NO.sub.x molecules with EDA constituent of stack gas. Richards describes using UV light having a wavelength of 150-500 nanometers between 300-400 nanometers to promote photo-oxidation of EDA complex. Richards suggests that free radical reactions may occur due to UV exposure (column 8, lines 12-15). A careful examination of the Richards patent reveals that (Table of column 9, lines 1-21) the underlying reaction mechanisms required for practicing his patent are similarly limited to those disclosed by Zenty as reactions 17-19 of U.S. Pat. No. 4,097,349. Again, Richards does not deal with fly-ash.
U.S. Pat. No. 5,138,175 to Kim, et al. describes irradiation of gas mixtures such as combustion gases and flue-gases to facilitate removal of sulfur and nitrogen oxide contaminants. Kim et al. demonstrates SO.sub.2 can be efficiently removed from flue-gases given sufficient exposure to UV light and presence of adequate amounts of oxygen and water. As far as the photo chemistry is concerned, this patent describes a method for the reduction of sulfur and nitrogen oxides in a gas mixture through UV radiation induced generation of ground state (zero charge atomic) oxygen and subsequent attack of such ground state O.sub.2 upon SO.sub.2 and NO.sub.x. Kim et al., further discloses that a source of radiation having a wavelength of, most desirably, below 220 nanometers installed within a dust-occluding air pressure window device located within the flue-gas stream. What is claimed by Kim, et al., is essentially a sheath design useful for in-situ treatment of flue-gas SO.sub.2 and NO.sub.2. The device supposedly extends the operating life of the lamp and to protect the surfaces of the lamp from fouling.
The patents above teach flue gas treatment by a known photochemical process involving UV radiation to generate highly oxidizing species that attack target molecules in the flue gas. They may differ on the postulated reaction mechanisms, but they all claim to be processes that remove or help remove gaseous pollutants from the flue gases. More importantly, none of the patents discussed above methods and apparatus based on hydroxyl radical reaction engineering and photo-system design.
Several U.S. patents involve in-situ flue-gas fly-ash conditioning and involve techniques for sulfur trioxide injection. U.S. Pat. Nos.: 3,993,429 to Archer; 4,333,746 to Southam; 5,320,052 to Spokoyny et al.; 5,350,441 and 5,196,038 to Wright; 5,229,077 to Bell et al. and U.S. Pat. Nos. 4,966,610 and 5,122,162 to Krigmont et al. (1992) involve various applications to control the addition of a reagent based on measurements of the feedstream and rely on sulfur trioxide injection components along with using Electrostatic Precipitators (ESP) components. However, none of these patents provides for the in-situ transformation of chemical species present in flue gas to form sulfur trioxide for conditioning fly ash.